1. Field of the Invention
The present invention relates to a blankmask for a photomask used to manufacture a semiconductor device, a photomask, and a method of manufacturing the photomask, and more particularly, to a blankmask for a photomask, which is thin and is formed of a metal film and a hard film having different reflectivities such that a critical dimension (CD) can be measured and a defect inspected therein, and a photomask using the blankmask.
2. Discussion of Related Art
As semiconductor devices have become finer, the performances and functions thereof, e.g., high operating speeds, low power consumption, and the like, have been improved and manufacturing costs have been reduced. Thus, there is a growing need to manufacture much finer semiconductor devices. Lithography technology has been used to manufacture a fine semiconductor device. Much attention has been paid to a transfer mask for performing the lithography technology, as critical technology for manufacturing a fine semiconductor device, together with an exposure device and resist materials.
Recently, semiconductor devices of the post-45-nm-half-pitch generation have been developed. A half-pitch of 45 nm is ⅕ of 193 nm that is a wavelength of exposure light of an ArF excimer laser. To manufacture a semiconductor device of the post-45-nm-half-pitch generation, it is insufficient to use only resolution enhancement technology (RET), such as conventional phase inversion and off-axis illumination, and optical proximity correction (OPC) technology. Thus, immersion exposure lithography technology or double patterning technology is required.
When a photomask having a light-shielding film pattern formed on a transparent substrate is manufactured, a resist film having a mask pattern is used as an etch mask to perform dry etching on a metal film. Thus, the resist film is also etched and consumed, thereby lowering the resolution of the photomask. To solve this problem, the metal film should be formed as a thin film. However, in this case, optical density of the photomask decreases.
To solve this problem, there has been developed a blankmask for a hardmask, in which a molybdenum silicide (MoSi)-based metal film and a chrome-based hard film are formed on a substrate. When the chrome-based hard film is used, a load on a resist film may be reduced and a resolution of the photomask may be prevented from being reduced when a mask pattern is transcribed onto the chrome-based hard film that is a thin film. The blankmask for a hardmask is used to etch a lower thin film using a hard film formed of an inorganic material as an etch mask, instead of a conventional resist film formed of an organic material. In this case, the hard film may be thinner than the conventional resist film, thereby improving an aspect ratio and an etch selectivity with respect to a metal film that is to be etched. Thus, when the metal film is etched using the hard film as an etch mask, a loading effect according to a pattern density and the distances between patterns may be reduced, thereby improving a critical dimension (CD) mean to target (MTT), CD linearity, CD uniformity, and the like.
When the blankmask for a hardmask described above is applied to manufacturing a device having a half-pitch of 32 nm or less and particularly, a half-pitch of 22 nm or less, the following problems may occur.
First, a hard film may have poor etch characteristics.
FIGS. 1A to 1D are cross-sectional views of a process of manufacturing a photomask using a conventional hard film. Referring to FIG. 1A, a blankmask in which a metal film 20, a hard film 30, and a resist film 40 are formed on a substrate 10 is formed. Then, as illustrated in FIG. 1B, the resist film 40 is patterned to form a resist pattern 40a, and an etch process is performed using the resist pattern 40a as an etch mask to form a hard film pattern 30a. Then, as illustrated in FIG. 1C, the metal film 20 is etched using the hard film pattern 30a as an etch mask to form a metal film pattern 20a. Referring to FIG. 1D, the hard film pattern 30a is removed, thereby completing the manufacture of the photomask.
However, since the hard film 30 has a thin thickness of several to several tens of, a problem, e.g. a CD bias, occurs when an etch rate of the hard film 30 is less than or greater than a desired level during an etch process. For example, if the etch rate of the hard film 30 is high, skew occurs, for example, a CD exceeds a target CD, and a CD is difficult to control. If the etch rate of the hard film 30 is too low, the thickness of the resist film 40 which is an upper film decreases and the loading effect occurs on the hard film 30. In particular, a CD of a desired device becomes finer. Accordingly, the etch characteristics of the hard film 30 are very important.
Second, resolution and pattern fidelity of a photomask are limited by the thickness of a metal film.
When a photomask is manufactured by patterning a metal film using a hard film as an etch mask to form a fine pattern having a half-pitch of 32 nm or less and particularly, a half-pitch of a 22 nm or less, the resolution and pattern fidelity of the photomask are influenced by the thickness of the metal film.
More specifically, as a half-pitch of a dynamic random access memory (DRAM) becomes fine, e.g., 45 nm or 32 nm, optical proximity correction (OPC) design for forming a fine pattern during manufacture of a photomask becomes more complex. When OPC is performed, a sub-resolution feature size (SRFS) in which a pattern is not substantially formed on a wafer according to an ×4 reduction exposure method requires a pattern of 60 nm on a photomask when a half-pitch of a DRAM is 45 nm and requires a pattern of 42 nm on the photomask when the half-pitch of the DRAM is 32 nm. If a metal film is thick, the SRFS is not precisely formed. Such a problem becomes worse when a DRAM has a half-pitch of 22 nm or less. To solve this problem, the metal film may be formed to be thin. However, the thickness of the metal film is directly related to the optical density thereof. For this reason, there are restrictions to forming the metal film to be thin.
Third, it is difficult to inspect a hard film and a metal film.
During a photomask process, an inspection of verifying processes of manufacturing a hard film and a metal film may be classified as a transmissive inspection method or a reflective inspection method as illustrated in FIGS. 2A and 2B. The method illustrated in FIG. 2A uses the difference between the transmissivity a of light passing through a metal film 20 and the transmissivity b of light passing through a hard film pattern 30a. The method illustrated in FIG. 2B uses the reflectivity a′ of light from a metal film 20 and the reflectivity b′ of light from a hard film pattern 30a. 
In the case of the transmissive inspection method, when the hard film pattern 30a is inspected, the difference between the transmissivities of the hard film pattern 30a and the metal film 20 which is a reference layer is 0.1% or less. Thus, since the transmissivity contrast between the metal film 20 and the hard film pattern 30a is substantially zero, the hard film pattern 30a cannot be practically inspected. Thus, the reflective inspection method is indispensable to inspecting the hard film pattern 30a. However, when the metal film 20 is formed to be thin so as to solve the problems described above, the reflectivity of light from the metal film 20 increases at an inspection wavelength. In this case, when the hard film pattern 30a having a thin thickness is used, the reflectivity contrast is lowered.